The invention relates generally to catheters and more particularly, to expandable catheters having electrodes for applying energy to biological tissue, and methods therefor.
The venous system contains a plurality of valves for directing blood flow back to the heart. The venous system of the lower limb consists essentially of the superficial venous system and the deep venous system with perforating veins connecting the two systems. The superficial system includes the great saphenous vein and the small saphenous vein. The deep venous system includes the anterior and posterior tibial veins which unite to form the popliteal vein which in turn becomes the femoral vein when joined by the small saphenous vein.
In FIG. 1 there is shown a partial cross-sectional view of a dilated vein 10 from a lower limb having competent valves 12. Venous valves are usually bicuspid valves, with each cusp forming a sack or reservoir 16 for blood which, under pressure, forces the free edges of the cusps together to prevent retrograde flow of the blood and allow only antegrade flow to the deep veins and heart. The arrow 18 leading out the top of the vein represents the antegrade flow of blood back to the heart. Properly operating venous valves prevent retrograde flow as blood is pushed forward through the vein lumen and back to the heart. When an incompetent valve 14 attempts to close in response to a pressure gradient across the valve, the cusps do not seal properly and retrograde flow of blood occurs. Venous insufficiency is a chronic disease involving the incompetence of venous valves.
Chronic venous insufficiency is a problem caused by hydrodynamic forces acting on the lowest part of the body: the legs, ankles and feet. As the veins dilate due to increased pressure, the retrograde flow of blood may occur and the valves in the veins become less able to withstand the weight of the blood above them. The weight of the blood causes the veins to dilate further and the valves in the veins to fail. Localized incompetence of a valve in a perforator vein allows reflux of blood from the deep venous system to the superficial venous system. Reflux may be expressed as the peak reverse velocity of blood as a function of vein diameter. Patients with phlebitis may have damaged vein valve leaflets.
Patients who develop chronic venous insufficiency of the lower extremities frequently develop complications of this disease, including skin discoloration, varicose veins, and ulcerations. These patients may develop blood clots in their legs which can travel to their lungs, resulting in a pulmonary embolism. These complications develop over time, with increasingly severe damage to the veins and the valves within the veins.
The varicose vein condition includes dilation and tortuosity of the superficial veins of the lower limbs, resulting in unsightly discoloration, pain, swelling, and possibly ulceration. Varicose veins often involve incompetence of one or more venous valves, which allow reflux of blood within the superficial system. This can also be worsened by deep venous reflux and perforator reflux. Current treatments include surgical procedures such as vein stripping, ligation, and occasionally, vein segment transplant, venous valvuloplasty, and the implantation of various prosthetic devices. The removal of varicose veins from the body can be a tedious, time-consuming procedure having a painful and slow healing process. In addition, patients with varicose veins may undergo injection sclerotherapy, or removal of vein segments. Complications, scarring, and the loss of the vein for future cardiac and other by-pass procedures may also result. Along with the complications and risks of invasive surgery, varicose veins may persist or recur, particularly when the valvular problem is not corrected. Due to the long, technically demanding nature of the surgical valve reconstruction procedure, treating multiple venous sections with surgical venous valve repair is rarely performed. Thus, a complete treatment of all important incompetent valves has been impractical.
Venous insufficiency often consists of hypertension of the lower limb in the deep, perforating and often superficial veins. Existing treatments for chronic venous insufficiency are often less than ideal. These treatments include the elevation of the legs, compressing the veins externally with elastic support hose, perforator ligation, surgical valve repair, and grafting vein sections with healthy valves from the arm into the leg. These methods have variable effectiveness. Moreover, invasive surgery has its associated complications with risk to life and expense. Similarly, the palliative therapies require major lifestyle changes for the patient. For example, the ulcers may recur unless the patient continues to elevate the legs and use pressure gradient stockings for long continuous periods of time.
Due to the time-consuming and invasive nature of the current surgical treatments, such as valvuloplasty or vein segment grafting, typically only one valve is treated during any single procedure. This greatly limits the ability of the physician to fully treat patients suffering from chronic venous insufficiency. Every instance of invasive surgery, however, has its associated complications with morbidity and expense.
Another type of treatment, the ligation of vascular lumina by cauterization or coagulation using electrical energy from an electrode, has been employed as an alternative to the surgical removal of superficial and perforator veins. However, such ligation procedures also close off the lumen, essentially destroying its functional capability. For example, it is known to introduce an electrode into the leg of a patient, and position the electrode adjacent the exterior of the varicose vein to be treated. Through a small stab incision, a probe is forced through the subcutaneous layer between the fascia and the skin, and then to the vein to be destroyed. A monopolar electrode at the outer end of the probe is placed adjacent the varicose vein and the return electrode is placed on the skin. Once properly positioned, an alternating current of 500 kHz is applied to destroy the adjacent varicose vein by electrocoagulation. The coagulated vein loses the function of allowing blood to flow through, and is no longer of use. For example, occluding or ligating the saphenous vein would render that vein unavailable for harvesting in other surgical procedures such as coronary by-pass operations.
Catheters having bowable or expandable arms with electrodes mounted on the arms may be used to apply energy to the inside surface of a hollow anatomical structure. In shrinking a vein for instance, it is desirable to apply energy evenly around the entire inner surface of the vein at the treatment location so that the full inner surface is evenly heated. The evenly-heated surface should then contract more uniformly to shrink the vein diameter. To apply energy to the vein wall, it is preferable to bring a plurality of evenly-spaced electrodes into apposition with the vein tissue. It is also preferable to use electrodes that are as wide as possible as the wider sized electrodes will be closer together when in apposition with the vein wall and will result in a more even application of energy to the vein wall.
However, having large electrodes on small catheters can increase the chances of shorting between those electrodes in which case no power will be applied to the target tissue. Bowable arms that have been made larger to support larger electrodes will allow less room at the anchor points of the arms to the catheter body causing them to be closer together which also provides less room for wiring the electrodes in the arms. Wiring is not only needed for energizing the electrodes on the bowable arms, but may also be needed for a temperature sensor mounted on an electrode or electrodes. Reducing the number of wires can greatly alleviate this concern.
Further considerations in the design of a reliable and effective bowable catheter for applying energy to a hollow anatomical structure include the control over forces that may be asymmetrical and that may tend to cause the arms to expand and contract so that they are not uniformly spaced. Additionally, improvements in the mounting of temperature sensors to the bowable arms may also increase effectiveness of the catheter.
Yet another consideration in the design of expandable catheters is the ability to provide a fluid flush or other useful fluid from the catheter or through a coaxial vascular sheath into the biological structure in which the catheter is used. Such fluids may be used to clear the biological structure of undesirable fluids, or to provide a radio-opaque fluid for a catheter location process, or for delivering therapeutic drugs, or for other reasons. However, applying a fluid from the catheter or a coaxial sheath to the biological structure may have the effect of lowering the temperature at the electrode or electrodes. Should that electrode or electrodes have a temperature sensor, the power control system connected to the catheter may mistakenly apply additional power to the electrode to increase the temperature of the biological structure, only to find that when the fluid flush is terminated, the temperature is now too high. The power control system must then terminate the application of power to the electrode on the arm. It would be desirable to avoid this form of power cycling when a fluid flush is applied by the catheter operator.
A consideration applicable to expandable catheters is the avoidance of fluid leakage into the catheter around movable parts. Another consideration is the avoidance of catheter distortion through use of those movable parts. For example, operating the expansion mechanism to control the expansion and contraction of the expandable arms may subject the catheter shaft to axial stresses that tend to undesirably lengthen or compress the catheter shaft. At the same time, it is desirable to maintain catheter shaft flexibility.
Hence, those skilled in the art have recognized the needs for an expandable electrode catheter that has increased electrode size while maintaining the catheter size as small as practical, in addition to providing improved control over forces that may tend to adversely affect the operation of the expandable arms as well as the catheter shaft. Additionally, those skilled in the art have recognized the need for an improved mounting technique for temperature sensors to the expandable arms as well as the avoidance of fluid leakage into the catheter around movable parts, while maintaining catheter shaft flexibility. Recognized also is the need for control over the power system coupled to the catheter so that unnecessary cycling does not occur when fluid flushes have been applied by the catheter operator. The invention fulfills these needs as well as others.